Semiconductor device, method of fabricating same, and electrooptical device

ABSTRACT

A pair of substrates forming the active matrix liquid crystal display are fabricated from resinous substrates having transparency and flexibility. A thin-film transistor has a semiconductor film formed on a resinous layer formed on one resinous substrate. The resinous layer is formed to prevent generation of oligomers on the surface of the resinous substrate during formation of the film and to planarize the surface of the resinous substrate.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to a configuration of thin-filmtransistors (TFTs) formed on a flexible substrate (i.e., havingmechanical flexibility) such as a resinous substrate which can be madeof engineering plastics. The invention also relates to a method offabricating such thin-film transistors. Furthermore, the inventionrelates to an active matrix liquid crystal display fabricated, usingthese thin-film transistors.

2. Prior Art

Thin-film transistors formed on glass substrates or on quartz substratesare known. Thin-film transistors formed on glass substrates are chieflyused in active matrix liquid crystal displays. Since active matrixliquid crystal displays can display images with high response and withhigh information content, it is expected that they can supplant simplematrix liquid crystal displays.

In an active matrix liquid crystal display, one or more thin-filmtransistors are disposed as a switching element at each pixel. Electriccharge going in and out of the pixel electrode is controlled by thisthin-film transistor. The substrates are made of glass or quartz,because it is necessary that visible light pass through the liquidcrystal display.

Liquid crystal displays are display means which are expected to findquite extensive application. For example, they are expected to be usedas display means for card-type computers, portable computers, andportable electronic devices for various telecommunication appliances. Asmore sophisticated information is treated, more sophisticatedinformation is required to be displayed on the display means used forthese portable electronic devices. For example, there is a demand forfunctions of displaying higher information content and moving picturesas well as numerals and symbols.

Where a liquid crystal display is required to have a function ofdisplaying higher information content and moving pictures, it isnecessary to utilize an active matrix liquid crystal display. However,where substrates made of glass or quartz are used, various problems takeplace: (1) limitations are imposed on thinning of the liquid crystaldisplay itself; (2) the weight is increased; (3) if the thickness isreduced in an attempt to reduce the weight, the substrate breaks; and(4) the substrate lacks flexibility.

Especially, card-type electronic devices are required to be so flexiblethat they are not damaged if slight stress is exerted on them when theyare treated. Therefore, liquid crystal displays incorporated in theseelectronic devices are similarly required to be flexible.

The invention disclosed herein provides an active matrix liquid crystaldisplay having flexibility.

SUMMARY OF THE INVENTION

One available method of imparting flexibility to a liquid crystaldisplay is to use plastic or resinous substrates which transmit light.However, because of poor heat resistance of resinous substrates, it istechnically difficult to form thin-film transistors on them.

Accordingly, the invention disclosed herein solves the foregoingdifficulty by adopting the following configuration:

One invention disclosed herein comprises: a filmy resinous substrate; aresinous layer formed on a surface of said resinous substrate; andthin-film transistors formed on said resinous layer.

A specific example of the above-described configuration is shown inFIG. 1. In the configuration shown in FIG. 1, a resinous layer 102 is incontact with a PET film 101 having a thickness of 100 μm, the PET filmbeing a filmy resinous substrate. Inverted-staggered thin-filmtransistors are formed on the resinous layer.

The material of the filmy resinous substrate can be selected from PET(polyethylene terephthalate), PEN (polyethylene naphthalate), PES(polyethylene sulfite), and polyimide. The requirements are flexibilityand transparency. Preferably, the maximum temperature that the materialcan withstand is made as high as possible. If the heating temperature iselevated above 200° C., oligomers (polymers having diameters of about 1μm) are generally deposited on the surface, or gases are produced.Therefore, it is quite difficult to form a semiconductor layer on theresinous substrate. Consequently, the material should have the highestpossible processing temperature.

In the above-described structure, the resinous layer acts to planarizethe surface of the resinous substrate. The planarization also serves toprevent precipitation of oligomers on the surface of the resinoussubstrate during steps involving heating such as the step for formingthe semiconductor layer.

The material of this resinous layer can be selected from methyl estersof acrylic acid, ethyl esters of acrylic acid, butyl esters of acrylicacid, and 2-ethylhexyl esters of acrylic acid. Even if resinoussubstrates are used, this resinous layer can suppress the drawbacks withfabrication of the afore-mentioned thin-film transistors.

The configuration of another invention comprises the steps of: forming aresinous layer on a filmy resinous substrate; forming a semiconductorlayer on said resinous layer by plasma-assisted CVD; and formingthin-film transistors, using said semiconductor layer.

The configuration of a further invention comprises the steps of:heat-treating a filmy resinous substrate at a given temperature to degassaid resinous substrate; forming a resinous layer on the filmy resinoussubstrate; forming a semiconductor layer on said resinous substrate byplasma-assisted CVD; and forming thin-film transistors, using saidsemiconductor layer.

In the above-described structure, heat-treatment is made to degas theresinous substrate, in order to prevent escape of gases from theresinous substrate during later processes involving heating. Forexample, if gases are released from the resinous substrate when asemiconductor thin film is being formed on the resinous substrate, thenlarge pinholes are formed in the semiconductor thin film. This greatlyimpairs the electrical characteristics. Accordingly, the substrate isheat-treated at a temperature higher than heating temperatures used inthe later processes, to degas the resinous substrate. In this way,release of gases from the resinous substrate during the later steps canbe suppressed.

The configuration of a yet other invention comprises the steps of:heat-treating a filmy resinous substrate at a given temperature; forminga resinous layer on said filmy resinous substrate; forming asemiconductor layer on said resinous substrate by plasma-assisted CVDwhile heating the substrate to a temperature lower than said giventemperature; and forming thin-film transistors, using said semiconductorlayer.

The configuration of a still other invention comprises the steps of:heat-treating a filmy resinous substrate at a given temperature which ishigher than any heat-treatment temperature used in other steps; forminga resinous layer on said filmy resinous substrate; forming asemiconductor layer on said resinous substrate by plasma-assisted CVD;and forming thin-film transistors, using said semiconductor layer.

The configuration of a still further invention comprises: a pair offilmy resinous substrates; a liquid crystal material held between saidresinous substrates; pixel electrodes formed on a surface of at leastone of said resinous substrates; thin-film transistors connected withsaid pixel electrodes and formed on said resinous substrate; andresinous layers formed on surfaces of said filmy resinous substrates toplanarize the surfaces.

A specific example of the above-described structure is shown in FIG. 3.In the structure shown in FIG. 3, a pair of resinous substrates 301,302, a liquid crystal material 309 held between these resinoussubstrates, pixel electrodes 306, thin-film transistors (TFTs) 305connected with the pixel electrodes 306, and a resinous layer 303 forplanarizing the surface of the resinous substrate 301.

BRIEF DESCRIPTION OF THE DRAWINGS

FIGS. 1(A) to 1(E) are views illustrating a process sequence forfabricating thin-film transistors according to the present invention;

FIGS. 2(A) to 2(C) are views illustrating another process sequence forfabricating thin-film transistors according to the present invention;and

FIG. 3 is a schematic cross-sectional view of a liquid crystal panel.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS Example 1

The present example shows an example in which inverted-staggered TFTsare formed on a substrate of PET (polyethylene terephthalate) which isan organic resin.

As shown in FIG. 1(A), a PET film 101 having a thickness of 100 μm isfirst prepared and heat-treated to degas the film. This heat-treatmentis required to be conducted at a temperature higher than the highesttemperature applied in later processes. In the processes shown in thepresent example, a temperature of 160° C. used during formation of anamorphous silicon film by plasma-assisted CVD is the highest heatingtemperature. Therefore, the heat-treatment for degassing the PET film isperformed at 180° C.

A layer of an acrylic resin 102 is formed on this PET film 101. As anexample, a methyl ester of acrylic acid can be used as the acrylicresin. This acrylic resin layer 102 acts to prevent precipitation ofoligomers on the surface of the PET film 101 in processes conductedlater. The acrylic resin layer 102 also serves to planarize the unevensurface of the PET film 102. Generally, PET film surface has unevennessof the order of several hundreds of angstroms to 1 μm. Such unevennessgreatly affects the electrical properties of the semiconductor layerhaving a thickness of several hundreds of angstroms. Therefore, it isquite important to planarize the base on which the semiconductor layeris formed.

Then, gate electrodes 103 of aluminum are formed. The gate electrodes103 are formed by forming an aluminum film to a thickness of 2000 to5000 Å (3000 Å in this example) by sputtering and performing awell-known patterning step making use of photolithography. The gateelectrodes 103 are etched so that the side surfaces are tapered (FIG.1(A)).

Thereafter, a silicon oxide film acting as a gate-insulating film 104 isformed to a thickness of 1000 Å by sputtering. The gate-insulating film104 may be made from silicon nitride instead of silicon oxide.

Subsequently, a substantially intrinsic (I-type) amorphous silicon film105 is formed to a thickness of 500 Å by plasma-assisted CVD under thefollowing conditions:

film formation temperature 160° C. (at which the substrate is heated):reaction pressure: 0.5 torr RF power (13.56 MHz): 20 mW/cm² reactantgas: SiH₄

In this example, the film is formed, using a parallel-plate plasma-CVDmachine. The substrate is heated by a heater disposed within a substratestage in which the resinous substrate is placed. In this way, the stateshown in FIG. 1(B) is obtained.

Then, a silicon oxide film which acts as an etch stopper in a later stepis formed by sputtering and then patterned to form an etch stopper 106.

Thereafter, an n-type amorphous silicon film 107 is formed to athickness of 300 Å by parallel-plate plasma-assisted CVD under thefollowing conditions:

film formation temperature 160° C. (at which the substrate is heated):reaction pressure: 0.5 torr RF power (13.56 MHz): 20 mW/cm² reactantgases: B₂H₆/SiH₄ = 1/100

In this way, the state shown in FIG. 1(C) is obtained. Then, the n-typeamorphous silicon film 107 and the substantially intrinsic (I-type)amorphous silicon film 105 are patterned by a dry-etching process. Analuminum film is formed to a thickness of 3000 Å by sputteringtechniques. Thereafter, this aluminum film and the underlying n-typeamorphous silicon film 107 are etched to form source electrodes 108 anddrain electrodes 109. During this etching process, the action of theetch stopper 106 assures that the source and drain regions are isolatedfrom each other (FIG. 1(D)).

An interlayer dielectric layer 110 is formed out of silicon oxide or aresinous material such as polyimide to a thickness of 6000 Å. Where asilicon oxide film is formed, a liquid which is applied when the siliconoxide film is formed may be used. Finally, contact holes are formed, andpixel electrodes 111 are fabricated from ITO. In this way, thin-filmtransistors arranged at the pixel electrodes of the active matrix liquidcrystal display can be fabricated, using the transparent resinoussubstrate (FIG. 1(E)).

Example 2

The present example shows a case in which an active matrix liquidcrystal display is fabricated, using the thin-film transistors describedin Example 1. The liquid crystal electrooptical device described in thepresent example is shown in FIG. 3 in cross section.

In FIG. 3, PET films 301 and 302 having a thickness of 100 μm form apair of substrates. An acrylic resin layer 303 acts as a planarizinglayer. Indicated by 306 are pixel electrodes. In FIG. 3, only thestructure corresponding to two pixels is shown.

Indicated by 304 is a counter electrode. Orientation films 307 and 308orient a liquid crystal 309 which can be a twisted-nematic (TN) liquidcrystal, supertwisted-nematic (STN) liquid crystal, or a ferroelectricliquid crystal. Generally, a TN liquid crystal is employed. Thethickness of the liquid crystal layer is several micrometers to about 10μM.

Thin-film transistors (TFTs) 305 are connected with the pixel electrodes306. Electric charge going in and out of the pixel electrodes 306 iscontrolled by the TFTs 305. In this example, only one of the pixelelectrodes 306 is shown as a typical one but a required number of otherconfigurations of similar structure are also formed.

In the structure shown in FIG. 3, the substrates 301 and 302 haveflexibility and so the whole liquid crystal panel can be made flexible.

Example 3

The present example shows an example in which coplanar thin-filmtransistors used for an active matrix liquid crystal display arefabricated. The process sequence for fabricating the thin-filmtransistors of the present example is shown in FIG. 2. First, a PET film201 having a thickness of 100 μm is prepared as a filmy organic resinsubstrate. The film is heated-treated at 180° C. to promote degassingfrom the PET film 201. A layer of an acrylic resin 202 is formed on thesurface of the film. In this example, an ethyl ester of acrylic acid isused as the acrylic resin.

Then, a substantially intrinsic (I-type) semiconductor layer 203 inwhich a channel formation region is formed is grown by plasma-assistedCVD under the following conditions:

film formation temperature 160° C. (at which the substrate is heated):reaction pressure: 0.5 torr RF power (13.56 MHz): 20 mW/cm² reactantgas: SiH₄In this example, a parallel-plate plasma-CVD machine is used to grow thefilm.

Then, an n-type amorphous silicon film is grown to a thickness of 300 Åby the parallel-plate plasma-CVD machine under the following conditions:

film formation temperature 160° C. (at which the substrate is heated):reaction pressure: 0.5 torr RF power (13.56 MHz): 20 mW/cm² reactantgases: B₂H₆/SiH₄ = 1/100

The n-type amorphous silicon film is patterned to form source regions205 and drain regions 204 (FIG. 2(A)).

A silicon oxide film or silicon nitride film acting as a gate-insulatingfilm is formed by sputtering techniques and patterned to form thegate-insulating film 206. Gate electrodes 207 are then formed fromaluminum (FIG. 2(B)).

A polyimide layer 208 is formed as an interlayer dielectric film to athickness of 5000 Å. Contact holes are formed. ITO electrodes 209becoming pixel electrodes are formed by sputtering, thus completing TFTs(FIG. 2(C)).

Example 4

The present example is similar to the structure of Example 1 or 2 exceptthat the semiconductor layer is made of a microcrystalline semiconductorfilm. First, a substantially intrinsic semiconductor layer is grown asthe microcrystalline semiconductor layer under the following conditions:

film formation temperature 160° C. (at which the substrate is heated):reaction pressure: 0.5 torr RF power (13.56 MHz): 150 mW/cm² reactantgases: SiH₄/H₂ = 1/30In this example, a parallel-plate plasma-CVD machine is used to grow thefilm.

The conditions under which an n-type microcrystalline silicon film isgrown are described below. Also in this case, a parallel-plateplasma-CVD machine is used.

film formation temperature 160° C. (at which the substrate is heated):reaction pressure: 0.5 torr RF power (13.56 MHz): 150 mW/cm² reactantgases: B₂H₆/SiH₄ = 1/100

Generally, a microcrystalline silicon film can be obtained by supplyingpower of 100 to 200 mW/cm². In the case of the I-type semiconductorlayer, desirable results are obtained by diluting silane with hydrogenby a factor of about 10 to 50, as well as by increasing the power.However, if the hydrogen dilution is made, the film growth rate drops.

Example 5

The present example relates to a method consisting of irradiating asilicon film with laser light having such a power that the filmy base orsubstrate is not heated, the silicon film having been formed byplasma-assisted CVD as described in the other examples.

A technique for changing an amorphous silicon film formed on a glasssubstrate into a crystalline silicon film by irradiating the amorphousfilm with laser light (e.g., KrF excimer laser light) is known. Inanother known technique, impurity ions for imparting one conductivitytype are implanted into the silicon film and then the silicon film isirradiated with laser light to activate the silicon film and theimpurity ions. The implantation of the impurity ions amorphizes thesilicon film.

The configuration described in the present example makes use of a laserirradiation process as described above, and is characterized in that theamorphous silicon film 105 shown in FIG. 1 or the amorphous siliconfilms 203 and 204 shown in FIG. 2 are irradiated with quite weak laserlight to crystallize the amorphous silicon film. If the previouslyformed film is a microcrystalline silicon film, the crystallinity can beimproved.

KrF excimer laser or XeCl excimer laser can be used to emit the laserlight. The energy of the emitted laser light is 10 to 50 mJ/cm². It isimportant that the resinous substrate 101 or 102 be not thermallydamaged.

By utilizing the invention disclosed herein, the thickness of an activematrix liquid crystal display can be reduced. Also, the weight can bedecreased. If an external force is applied, the substrates do not break.Flexibility can be imparted to the display.

This liquid crystal display can find wide application and is quiteuseful.

1. A portable electronic device comprising: a resinous substratecomprising polyethylene naphthalate; a thin film transistor formed overthe resinous substrate with a resinous layer interposed therebetween,the thin film transistor including a channel forming region and a gateelectrode with a gate insulating film interposed therebetween; and asilicon oxide film covering the thin film transistor, wherein thesilicon oxide film is formed by applying a liquid.
 2. A portableelectronic device comprising: a flexible substrate comprisingpolyethylene naphthalate; a thin film transistor formed over theflexible substrate with a resinous layer interposed therebetween, thethin film transistor including a channel forming region and a gateelectrode with a gate insulating film interposed therebetween; and asilicon oxide film covering the thin film transistor, wherein thesilicon oxide film is formed by applying a liquid.
 3. A portableelectronic device comprising: a resinous substrate comprisingpolyethylene naphthalate; a first and a second thin film transistorsformed over the resinous substrate with a resinous layer interposedtherebetween, each of the first and the second thin film transistorsincluding a channel forming region comprising crystalline silicon and agate electrode with a gate insulating film interposed therebetween; anda silicon oxide film covering the first and the second thin filmtransistors, wherein the silicon oxide film is formed by applying aliquid.
 4. A portable electronic device comprising: a flexible substratecomprising polyethylene naphthalate; a first and a second thin filmtransistors formed over the flexible substrate with a resinous layerinterposed therebetween, each of the first and the second thin filmtransistors including a channel forming region comprising crystallinesilicon and a gate electrode with a gate insulating film interposedtherebetween; and a silicon oxide film covering the first and the secondthin film transistors, wherein the silicon oxide film is formed byapplying a liquid.
 5. A portable electronic device comprising: aresinous substrate comprising polyethylene naphthalate; a thin filmtransistor formed over the resinous substrate with a resinous layerinterposed therebetween; and a silicon oxide film covering the thin filmtransistor, wherein the silicon oxide film is formed by applying aliquid, wherein the thin film transistor includes a channel formingregion comprising a crystalline silicon and a gate electrode with a gateinsulating film interposed therebetween, and wherein the crystallinesilicon is formed by laser irradiation.
 6. A portable electronic devicecomprising: a flexible substrate comprising polyethylene naphthalate; athin film transistor formed over the flexible substrate with a resinouslayer interposed therebetween; a thin film transistor formed over theresinous layer; and a silicon oxide film covering the thin filmtransistor, wherein the silicon oxide film is formed by applying aliquid, wherein the thin film transistor includes a channel formingregion comprising a crystalline silicon and a gate electrode with a gateinsulating film interposed therebetween, and wherein the crystallinesilicon is formed by laser irradiation.
 7. A portable electronic devicecomprising: a pair of resinous substrates opposing to each other,wherein the pair of resinous substrates comprise polyethylenenaphthalate; a resinous layer formed on at least one of the pair ofresinous substrates; a thin film transistor formed between the pair ofresinous substrates and formed on the resinous layer, the thin filmtransistor including a channel forming region and a gate electrode witha gate insulating film interposed therebetween; and a silicon oxide filmcovering the thin film transistor, wherein the silicon oxide film isformed by applying a liquid.
 8. A portable electronic device comprising:a pair of flexible substrates opposing to each other, wherein the pairof flexible substrates comprise polyethylene naphthalate; a resinouslayer formed on at least one of the pair of flexible substrates; a thinfilm transistor formed between the pair of flexible substrates andformed on the resinous layer, the thin film transistor including achannel forming region and a gate electrode with a gate insulating filminterposed therebetween; and a silicon oxide film covering the thin filmtransistor, wherein the silicon oxide film is formed by applying aliquid.
 9. A portable electronic device comprising: a pair of resinoussubstrates opposing to each other, wherein the pair of resinoussubstrates comprise polyethylene naphthalate; a resinous layer formed onat least one of the pair of resinous substrates; a thin film transistorformed between the pair of resinous substrates and formed on theresinous layer, the thin film transistor including a channel formingregion comprising crystalline silicon and a gate electrode with a gateinsulating film interposed therebetween; and a silicon oxide filmcovering the thin film transistor, wherein the silicon oxide film isformed by applying a liquid.
 10. A portable electronic devicecomprising: a pair of flexible substrates opposing to each other,wherein the pair of flexible substrates comprise polyethylenenaphthalate; a resinous layer formed on at least one of the pair offlexible substrates; a thin film transistor formed between the pair offlexible substrates and formed on the resinous layer, the thin filmtransistor including a channel forming region comprising crystallinesilicon and a gate electrode with a gate insulating film interposedtherebetween; and a silicon oxide film covering the thin filmtransistor, wherein the silicon oxide film is formed by applying aliquid.
 11. A portable electronic device comprising: a pair of resinoussubstrates opposing to each other, wherein the pair of resinoussubstrates comprise polyethylene naphthalate; a resinous layer formed onat least one of the pair of resinous substrates; a thin film transistorformed between the pair of resinous substrates and formed on theresinous layer; and a silicon oxide film covering the thin filmtransistor, wherein the silicon oxide film is formed by applying aliquid, wherein the thin film transistor includes a channel formingregion comprising a crystalline silicon and a gate electrode with a gateinsulating film interposed therebetween, and wherein the crystallinesilicon is formed by laser irradiation.
 12. A portable electronic devicecomprising: a pair of flexible substrates opposing to each other,wherein the pair of flexible substrates comprise polyethylenenaphthalate; a resinous layer formed on at least one of the pair offlexible substrates; a thin film transistor formed between the pair offlexible substrates and formed on the resinous layer; and a siliconoxide film covering the thin film transistor, wherein the silicon oxidefilm is formed by applying a liquid, wherein the thin film transistorincludes a channel forming region comprising a crystalline silicon and agate electrode with a gate insulating film interposed therebetween, andwherein the crystalline silicon is formed by laser irradiation.
 13. Aportable electronic device according to any one of claims1,2,3,4,5,6,7,8,9,10,11,12, wherein the portable electronic device is aportable computer.
 14. A portable electronic device according to any oneof claims 3,4,9,10, wherein the laser light comprises at least oneselected from the group consisting of KrF excimer laser light and XeCllaser light.
 15. A portable electronic device according to any one ofclaims 1,3,5,7,9,11, wherein the resinous substrate comprises a plasticsubstrate.
 16. A portable electronic device according to any one ofclaims 4,6,8,10,12, wherein the flexible substrate comprises a plasticsubstrate.
 17. A portable electronic device according to any one ofclaims 1,2,3,4,5,6,7,8,9,10,11,12, wherein the resinous layer comprisesan acrylic resin.
 18. A portable electronic device according to any oneof claims 1,2,3,4,5,6,7,8,9,10,11,12, wherein the resinous layercomprises at least one selected from the group consisting of methylesters of acrylic acid, ethyl esters of acrylic acid, butyl esters ofacrylic acid, and 2-ethylhexyl esters of acrylic acid.
 19. A portableelectronic device according to any one of claims1,2,3,4,5,6,7,8,9,10,11,12, wherein the thin film transistor comprisesan inverted-staggered thin-film transistor.
 20. A portable electronicdevice according to any one of claims 1,2,3,4,5,6,7,8,9,10,11,12,wherein the thin film transistor comprises a coplanar thin-filmtransistor.